Glycation and/or oxidation of LDL may promote diabetic nephropathy. The mitogen-activated protein kinase (MAPK) cascade, which includes extracellular signal-regulated protein kinases (ERKs), modulates cell function. Therefore, we examined the effects of LDL on ERK phosphorylation in cultured rat mesangial cells. In cells exposed to 100 microg/ml native LDL or LDL modified by glycation, and/or mild or marked (copper-mediated) oxidation, ERK activation peaked at 5 min. Five minutes of exposure to 10-100 microg/ml native or modified LDL produced a concentration-dependent (up to sevenfold) increase in ERK activity. Also, 10 microg/ml native LDL and mildly modified LDL (glycated and/or mildly oxidized) produced significantly greater ERK activation than that induced by copper-oxidized LDL +/- glycation (P < 0.05). Pretreatment of cells with Src kinase and MAPK kinase inhibitors blocked ERK activation by 50-80% (P < 0.05). Native and mildly modified LDL, which are recognized by the native LDL receptor, induced a transient spike of intracellular calcium. Copper-oxidized (+/- glycation) LDL, recognized by the scavenger receptor, induced a sustained rise in intracellular calcium. The intracellular calcium chelator (EGTA/AM) further increased ERK activation by native and mildly modified LDL (P < 0.05). These findings demonstrate that native and modified LDL activate ERKs 1 and 2, an early mitogenic signal, in mesangial cells and provide evidence for a potential link between modified LDL and the development of glomerular injury in diabetes.

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